This section provides background information related to the present disclosure which is not necessarily prior art.
Since the development of the first commercial lithium ion (Li-ion) battery in 1990, Li-ion batteries have been extensively studied by researchers all over the world. Due to the advantages of light weight, high energy density, and long cycle life, lithium ion batteries have been widely used in cell phones, lap-top computers, etc. However, for usage in hybrid, plug-in hybrid and full electric vehicles, Li-ion batteries that can provide even higher energy density and power capability, longer cycling and calendar life, and better safety are needed.
In recent years, due to its high specific capacity, there has been much interest in lithium-rich, metal oxide cathode materials, which can be represented as Li[M1-xLix]O2 or Li2MnO3.LiMO2 (M=Ni, Co, Mn). For example, Li[Li0.2Mn0.54Ni0.13Co0.13]O2 can deliver an initial discharge capacity as high as 250 mAh/g when cycled between 4.8 V to 2.0 V at 18 mA/g at room temperature. In this series of materials, lithium partially substitutes for transition metal ions and forms super lattice ordering or “Li2MnO3-like regions.” Powder X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and magic-angle-spinning nuclear magnetic resonance (MAS NMR) have shown LiMn6 cation ordering in the transition metal layers of these lithium-rich materials, and this is the characteristic atom arrangement of Li2MnO3. Li2MnO3 is electrochemically inactive between 4.5 V and 3.0 V, and it is believed to stabilize the electrochemically active LiMO2 component by maintaining the cathode structure and to improve the discharge capability by extracting lithium concomitant with release of oxygen (a net loss of Li2O) to form MnO2 at high potential.
LiMn2O4 has the spinel structure with lithium ions in the 8a tetrahedral sites and leaving the 16c octahedral sites empty. LiMn2O4 can not only de-intercalate one unit of Li from the 8a tetrahedral sites per formula at 4 V, but also can intercalate an additional unit of Li into the 16c octahedral sites at 3 V, resulting in a theoretical capacity of 296 mAh/g. However, the cycling stability of LiMn2O4 is poor in the 3 V region due to Jahn-Teller distortion.
Though oxygen loss leads to a high discharge capability, lithium-rich materials suffer from an undesirable huge irreversible capacity loss (ICL), which can be about 40 mAh/g to about 100 mAh/g in the first cycle when charged to 4.8 V. Many efforts have been made to reduce the ICL. For example, acid treatment has been an effective method for improving the electrochemical performance of cathode materials. However, it can adversely affect cycling stability and rate capability of the cathodes.
In recent years, composite cathodes have been developed. These composites are blends of lithium-rich material with lithium insertion hosts. In these composites, the lithium insertion hosts act to accommodate the lithium ions that could not be inserted back into layered lattices after the first charge. Although some ICL has been reduced by blending lithium-rich material with lithium insertion hosts, the composite materials exhibit some capacity fade during cycling. Accordingly, there remains a need to improve composite cathodes.